One problem related to engine knocking noise has been reported for the 2011 Toyota Prius. The most recently reported issues are listed below.
Upon starting vehicle, a very loud knocking noise, accompanied by shaking occurred. At this time no failure/warning lights appeared on dashboard. Moved car forward approximately 15 feet, and placed car in park, knocking/shaking did not abate. Turned car off. The next day, the car, upon starting had the same loud knocking/shaking. It was audible from several feet away from the car. On the second time, it was accompanied by a warning light to put the car in park. Vehicle was towed to the dealership.